150
Electron
acceptors
Oxygen
Nitrate,
metals
Sulfate
C02,
selected
C, - C2
compounds
S. Wakeham
Bacterial process;
Representative chemical reaction(s)
Aerobic respiration;
(CH zOlx(NH3)y(H3 P04)z + xOz -+ xCOz + xHzO + yNH 3 + Z H3P04
Nitrification:
NH: + 202 -+ NO; + HzO + 2H+
Sulfide Oxidation:
HS' + 20 2 -+ SO~' + H+
Denitrification:
SCH20 + 4NO; -+ 4HCO; + CO2 + 2N2 +3HzO
Manganese oxide reduction: 2+
_
CHzO + 2MnOz + 3C02 + H20 -+ 2Mn + 4HC03
Nitrate reduction:
2CH20 + NO; + 2H+ -+ 2C02 + NH: + H20
Iron oxide reduction:
CHzO + 4~OHh + 7COz -+ 8HCO; + 3HzO • 4Fe 2+
Sulfata reduction (fermentation):
2CH3CHOCOOH + SO!- -+ 2CH3COOH + 2HCO; + HzS
CH4 + SO~· -+ HCO; + HS' + HzO
4H2 + SO!· -+ HS' + OH- + 3HzO
Acetate fermentation:
CH3COOH -+ CH 4 + CO2
C02 reduction:
CO2 + 4H2 -+ CH4 + 2H20
Fig. 6.2. A schematic of zones of organic matter degradation and respiratory functions of bacteria that
occur during diagenesis of organic matter in sediments (from Deming and Baross 1993)
by bacteria. Recently, the mesopore hypothesis has given way to more discrete organic
aggregates on mineral surfaces (Ransom et al.1997, 1998; Mayer 1999). In either case,
there appears to be a threshold below which sorbed organic matter cannot be attacked
by exoenzymes. On the other hand, a surprisingly high proportion of organic matter
sorbed to particles is reversibly bound. Experiments have shown that once desorbed,
this organic matter is remarkably susceptible to microbial degradation (Wang and Lee
1993; Keil et al. 1994a). In addition, there are clear compositional variations as a function of particle size (Keil et al. 1994b, 1998; Bergamaschi et al. 1997) which may affect
DC behaviour. For example, vascular plant-derived lignin phenols are enriched in the
larger grain sizes consistent with plant debris being relatively large, while the finer
sizes are characterized by higher ratios of vanillic acid/vanillin, indicating that the finer
particles are more highly degraded. Hydrodynamic sorting of particulate matter on
continental shelves results in selective deposition of vascular plant material in midshelf sediments, but little is transported further off-shore. Thus, the physical association of organic matter with mineral particles plays an important role in the degradation, preservation, and transport of organic carbon.
Of the bacterially-mediated chemical reactions depicted in Fig. 6.2, the most energetically favourable for bacteria are those in which oxygen is the electron acceptor. It
follows that the extent of DC degradation (and preservation) in sediments is strongly
controlled by the average time that DC-containing particles are exposed to pore water
oxygen or the oxygen exposure time (DET) (Hartnett et al. 1998; Hedges et al. 1999).
Electron
acceptors
Oxygen
Nitrate,
metals
Sulfate
C02,
selected
C, - C2
compounds
S. Wakeham
Bacterial process;
Representative chemical reaction(s)
Aerobic respiration;
(CH zOlx(NH3)y(H3 P04)z + xOz -+ xCOz + xHzO + yNH 3 + Z H3P04
Nitrification:
NH: + 202 -+ NO; + HzO + 2H+
Sulfide Oxidation:
HS' + 20 2 -+ SO~' + H+
Denitrification:
SCH20 + 4NO; -+ 4HCO; + CO2 + 2N2 +3HzO
Manganese oxide reduction: 2+
_
CHzO + 2MnOz + 3C02 + H20 -+ 2Mn + 4HC03
Nitrate reduction:
2CH20 + NO; + 2H+ -+ 2C02 + NH: + H20
Iron oxide reduction:
CHzO + 4~OHh + 7COz -+ 8HCO; + 3HzO • 4Fe 2+
Sulfata reduction (fermentation):
2CH3CHOCOOH + SO!- -+ 2CH3COOH + 2HCO; + HzS
CH4 + SO~· -+ HCO; + HS' + HzO
4H2 + SO!· -+ HS' + OH- + 3HzO
Acetate fermentation:
CH3COOH -+ CH 4 + CO2
C02 reduction:
CO2 + 4H2 -+ CH4 + 2H20
Fig. 6.2. A schematic of zones of organic matter degradation and respiratory functions of bacteria that
occur during diagenesis of organic matter in sediments (from Deming and Baross 1993)
by bacteria. Recently, the mesopore hypothesis has given way to more discrete organic
aggregates on mineral surfaces (Ransom et al.1997, 1998; Mayer 1999). In either case,
there appears to be a threshold below which sorbed organic matter cannot be attacked
by exoenzymes. On the other hand, a surprisingly high proportion of organic matter
sorbed to particles is reversibly bound. Experiments have shown that once desorbed,
this organic matter is remarkably susceptible to microbial degradation (Wang and Lee
1993; Keil et al. 1994a). In addition, there are clear compositional variations as a function of particle size (Keil et al. 1994b, 1998; Bergamaschi et al. 1997) which may affect
DC behaviour. For example, vascular plant-derived lignin phenols are enriched in the
larger grain sizes consistent with plant debris being relatively large, while the finer
sizes are characterized by higher ratios of vanillic acid/vanillin, indicating that the finer
particles are more highly degraded. Hydrodynamic sorting of particulate matter on
continental shelves results in selective deposition of vascular plant material in midshelf sediments, but little is transported further off-shore. Thus, the physical association of organic matter with mineral particles plays an important role in the degradation, preservation, and transport of organic carbon.
Of the bacterially-mediated chemical reactions depicted in Fig. 6.2, the most energetically favourable for bacteria are those in which oxygen is the electron acceptor. It
follows that the extent of DC degradation (and preservation) in sediments is strongly
controlled by the average time that DC-containing particles are exposed to pore water
oxygen or the oxygen exposure time (DET) (Hartnett et al. 1998; Hedges et al. 1999).